1. Field of the Invention
The invention relates to the purification of waste water and more particularly relates to methods of removing cyanide and metal ions from aqueous solutions.
2. Brief Description of the Prior Art
Electroplating wastes present a variety of problems in the design and operation of efficient waste water treatment systems. A major difficulty results from the use of strong metal complex formers in the aqueous plating baths, the most widely used being cyanides; see Durney, L. J., Electroplating Engineering Handbook, Van Nostrand Reinhold Co., Inc., New York, N.Y., 4th Edition, 1984.
The removal of cyanides is of primary concern since cyanide itself is toxic and, due to its complexing ability, interferes with the removal of metals as well; see Nemerow, N. L., Industrial Water Pollution, Addison-Wesley, Inc., Reading, Mass., 1978.
In theory, cyanides may be removed by conversion to cyanates or CO.sub.2 and N.sub.2 (total destruction), or by distillation. Although a variety of researchers have demonstrated the ability of ozone, hydrogen peroxide and polysulfides for destruction or conversion and ion exchange for removal, the practical method of choice for cyanide removal has remained chlorine treatment; see for example Nemerow, supra., and Cherry, K. F., Plating Waste Treatment, Ann Arbor Science, Inc., Ann Arbor, Mich., 1982.
Removal of cyanides by distillation has not gained widespread acceptance in industrial practice, nor is it used to remove metal cations. Representative of this method of cyanide removal is the description given in the U.S. Pat. No. 3,744,977.
The present invention offers improvements and advantages over the prior art methods for removing cyanide and metallic ions from solution in water. The method of the invention comprises a waste water recovery process which exhibits many advantages over traditional cyanide destruction processes currently in use. These advantages include:
1. More complete removal and separation of cyanide from metal cations; the latter in turn being more efficiently removed in the uncomplexed form.
2. No addition of sodium or calcium ions to the waste water from hypochlorite since hypochlorite is not used. Also, no sodium chloride from the product of hypochlorite reaction.
3. Recovered alkali metal cyanide is reuseable.
4. The waste water need not be boiled for removal of hydrogen cyanide.
Other advantages of the method of the invention will be appreciated by those skilled in the art from the following description.